1. Field of the Invention
The invention relates to an injection cylinder in an injection apparatus for molding a metal material, wherein a molten metal material is injected into a mold from a front nozzle by using a plunger.
2. Description of the Related Art
Injection apparatuses that have been used for molding a metal material are classified into screw type and plunger type. Both types have an injection cylinder of almost the same basic configuration, with only differences in the means for melting and injecting the metal material.
Screw type injection apparatuses have a cylinder body that has a nozzle member attached to its front end and a feed opening formed in its rear top. Powdered metal material charged through the feed opening is heated to melt before it reaches the front part of the cylinder body by screw rotation. The molten material is measured (accumulated) in the front part of the cylinder body by retreating the screw, and then injected into a mold from the front nozzle by advancing the screw. (See International Patent Publication No. WO 90/09251)
Some plunger type injection apparatuses have a cylinder body that melts and stores a large amount of metal material inside. A portion of the molten material is measured (accumulated) in the front part of the cylinder body by retreating the plunger before injected out of the nozzle by advancing the plunger. In others, the metal material is melted in a melting furnace or other unit before fed into the cylinder body. In this melt feed method, the feed opening is formed in the top of the front part of the cylinder body.
Since metal materials require high temperatures above their liquidus curve to melt (for example, 595° C. or above for magnesium alloys), the relevant components of the injection cylinder are made of high-tension steel products having excellent high-temperature strength. For the sake of improved wear resistance on the cylinder's inner surface where the screw moves back and forth, some cylinder bodies have a cylinder liner inserted inside which is made of an alloy having higher wear resistance than that of steel products. (See Japanese Patent Application Laid-Open No. 2004-50248)
Possible means for attaching a cylinder liner to inside of a cylinder body include a shrink fit, as well as a tight fit where the cylinder liner is inserted at room temperatures before brought into close contact with the inner surface of the cylinder by thermal expansion. In either case, the cylinder liner is polished in the inner surface for increased bore diameter when the inner surface of the cylinder liner wears out after repetitive molding. A screw or plunger corresponding to that inside diameter is then selected to continue operation. With a shrink fit, the cylinder liner is difficult to pull out. Both the cylinder body and the cylinder liner are thus replaced with new ones when worn out beyond use limit.
For a tight fit, the cylinder liner is made of a material having a coefficient of thermal expansion higher than that of the material of the cylinder body. The cylinder liner is inserted into the cylinder body at room temperatures, and is expanded together with the cylinder body by the heating upon starting molding so that the cylinder liner comes into close contact with the cylinder's inner surface due to a difference between their coefficients of thermal expansion. In other words, when the two members cool down to room temperatures and the thermal expansion disappears, they restore their original sizes which facilitate pulling the cylinder liner out. This provides the economical advantage that the cylinder liner can be removed when the wearing in the inner surface reaches the use limit, and a new cylinder liner can be inserted into the cylinder body for use.
The close contact between the cylinder liner and the cylinder's inner surface is based on the difference between the coefficients of thermal expansion of their respective materials. A gap can thus result from insufficient contact if the inside diameter of the cylinder and the outside diameter of the cylinder liner have too large a tolerance. Meanwhile, too small a tolerance makes the insertion of the cylinder liner into the cylinder body before the thermal expansion operation so tight that the two members suffer unnecessary stress from thermal expansion. The application of injection pressure upon each molding also contributes to a drop in durability, shortening the life of the injection cylinder easily.
In the screw type apparatuses where the feed opening is formed in the rear part of the cylinder body, the metal material is yet to be melted and is in a solid state when in the vicinity of the feed opening. This means no penetration of the molten metal material from around the feed opening into between the cylinder liner and the cylinder's inner surface. In the plunger type apparatuses, on the other hand, the feed opening is arranged in the top of the front part where to melt and feed the metal material. The molten metal material can thus penetrate from around the feed opening into between the cylinder liner and the inner surface of the cylinder body because of injection pressure, and can even leak out from the contacting surfaces between the cylinder body and the nozzle member. For this reason, the cylinder body and the nozzle member are coupled to each other with a seal ring between their contacting surfaces. Making a plastic deformation for sealing, this seal ring is prone to degradation and requires replacement each time doing maintenance on the cylinder, the cylinder liner, the nozzle, the plunger, etc.